1. Field of the Invention
The invention relates to a photosensitive composition comprising a polymer and a photoactive constituent, as well as to a method of producing relief structure elements from this type of a photosensitive compound.
2. Description of Related Art
Structural elements in the .mu.m- and sub .mu.m range, which are photolithographically produced from photosensitive compounds, i.e. photoresists, play a key role in manufacturing highly integrated circuits. Namely, these types of structural elements generally serve as photomasks during a series of processing steps, such as ion implantation, doping, wet and dry etching, which are performed during the production of these components.
While the thermal and mechanical properties as well as the etch resistance of the relief structural elements can be influenced by the selection of the resin base--i.e. of the polymer--of the resist, the photostructurability is predominantly determined by the photoactive constituent. However, photoactive constituent and polymer must thereby match insofar as that, for example, segregation of the photosensitive compound cannot occur and that the exposed resist can be developed. Moreover, the photoactive constituent must satisfy a series of requirements such as free solubility, storage stability, ability to be developed with atoxic, highly aqueous--i.e., environmentally safe--developers, and high sensitivity--i.e., short exposure time--in conjunction with a bleaching of the photoactive constituent. Furthermore, the adhesion of the polymer to the substrate may not be impaired and an accurate transfer of the pattern, in conjunction with good resolution, must be guaranteed.
Either negative resists or positive resists can be used to produce fine structural elements (c.f. e.g.: "Angew. Chem.", Vol. 94 (1982), pp 471 to 485). Negative resists, in which the structuring principle is generally based on a cross-linkage of the exposed parts, to which a reduction of solubility is connected, exhibit a lower resolution, which is caused by swelling, in comparison to positive resists, and are usually developed with environmentally and economically unsound organic developers. In the case of positive resists, where the solubility in the developer is increased by means of the radiative action, the difference in solubility between the exposed and unexposed parts is usually brought about by a photochemically induced change of the polarity, whereby an aqueous alkaline developing process without swelling is made possible.
Commercially sold positive resists are generally based on novolak resins as an alkali-soluble base polymer and sulfonic esters of diazo naphthoquinones as a photoactive constituent (Ibid., p. 481). These types of resists satisfy the requirements regarding good bleaching properties, high sensitivity and high resolution down to the half-micron range during exposure in near UV, i.e., at exposure wavelengths of 436 or 365 nm, for example. In accordance with the increasing integration densities in electronic components, however, a resolution in the sub .mu.m range is required. This maximum resolution can be realized according to the formula CD=k .lambda./NA --where CD equals the smallest imaged structural element; .lambda. equals the wavelength of the light used for the lithographic process; NA equals the numerical aperture which is dependent on the optical system; and k equals a process-specific factor--by reducing the exposure wavelength, e.g. to 248 nm (DUV, KrF-Excimer laser). However, the aforesaid commercial resist systems cannot be used at this wavelength due to the insufficient bleaching properties of the photoactive constituents and the high absorption of the novolak base resins: namely, they have residual absorption values of &gt;0.3 .mu.m.sup.-1, whereas residual absorption values &lt;0.1 .mu.m.sup.-1 are required. Thus, suitable DUV-transparent base polymers and completely bleaching photoactive constituents which have the aforesaid profile of requirements are necessary to realize highest resolution, positively operating DUV-resists (DUV=Deep UV), in particular for an exposure at 248 nm.
Various 2-diazo-1,3-diketones have already been proposed as photoactive constituents (PAC) for positive DUV photoresists, which--analogous to the diazo naphthoquinones for the NUV-range (NUV=near UV)--are converted into corresponding carboxylic acids during DUV exposure via the Wolff-rearrangement process and the reaction of the thereby formed ketene with water, thereby making the resist able to be developed with alkalis in the exposed regions. Thus, the use of diazomeldrumic acid (2,2-dimethyl-5-diazo-1,3-dioxane-4,6-dione) and derivatives thereof is described as a PAC in mixtures using novolaks as a DUV-resist (c.f. U.S. Pat. No. 4,339,522 as well as "IEEE Trans. Electron Devices", Vol. ED-28 (1981), pp 1300-1305, and "J. Vac. Sci. Technol. B", Vol. 6 (1988), pp 559 to 563). Although these compounds exhibit very good bleaching properties in the DUV range in conjunction with high contrast and relatively high sensitivity (50 mJ/cm.sup.2), the resolution is limited to approximately 2 .mu.m due to the evaporation of the diazomeldrumic acid from the resist layer during the drying process and due to the poor solubility-inhibiting properties of this compound. Furthermore, no non-volatile derivatives of diazomeldrumic acid could be found with sufficient solubility in the usual resist solvents.
It is known from the teachings of U.S. Pat. Nos. 4,622,283 and 4,624,908 to use diazodimedone (2-diazo-5,5-dimethyl cyclohexane-1,3 dione) or derivatives thereof and other 2-diazo-1,3-diketones such as diazo-cyclopentandione as a DUV-PAC in mixtures with novolaks, polyvinylphenol and methacrylate/methacrylic acid copolymers (also c.f.: "Proc. of SPIE", Vol. 920 (1988), pp 51 to 58). Although, in contrast to diazomeldrumic acid and also to diazodimedone itself, the diazodimedone derivatives do not evaporate during the drying process and are soluble in resist solvents; just like diazodimedone and diazomeldrumic acid, they also exhibit excellent bleaching properties in the DUV range. However, due to the high dark film loss rates of the selected resist systems (14 to 42% of the original film thickness), only a resolution of 0.75 .mu.m at simultaneously sloped resist side-walls can be attained with these derivatives. Furthermore, these resist systems are relatively insensitive (100 to 150 mJ/cm.sup.2).
Additional non-volatile diazoketones with good DUV bleaching properties are diazotetramic acid (3-diazopyrrolidine-2,4-dione) and diazo homotetramic acid (3-diazopiperidine-2,4-dione) as well as 1,2-diphenyl-4-diazopyrazolidine-3,5-dione (c.f.: "Proc. of SPIE", Vol. 771 (1987), pp 2 to 10). These compounds serve as DUV-PAC's in resist formulas with novolak, styrene/maleinimide copolymers, polyglutarimide and polyvinylphenol. Also, structural elements with very sloped side-walls, which can perhaps be used in a three-layer system but which are unsuitable for sub-half micron lithography, are obtained with these systems due to the low contrast. Thus, 1-.mu.m-line/space patterns with very sloped side-walls are obtained after a polychromatic DUV exposure, and greatly overexposed 1.5-.mu.m-line/space patterns with somewhat steeper side-walls are obtained after exposure to a KrF-Excimer laser.
In addition, a system consisting of a chlorosulfonylphenyl-diazo cycloheptane dione derivative as a PAC and a styrene/maleic semi-ester copolymer as a base resin were already reported ("SPIE's 1989 Symposium on Microlithography", Feb. 26, 1989 to Mar. 3, 1989, San Jose, Calif., USA: Symposium Minutes, p 34). However, this system might not be able to be used in maximum resolution silicon technology both due to the corrosiveness of the chlorosulfonyl groups as well as the related storage instability under the influence of atmospheric humidity.
It is an object of the present invention to provide positively operating DUV photoresists, in particular for KrF-Excimer laser exposures at 248 nm, which, in the form of photosensitive compositions consisting of a polymer and a photoactive constituent, are able to be developed with aqueous, alkaline agents, where the photoactive constituents have good bleaching properties in the DUV range in conjunction with good solubility-inhibiting properties and where they do not evaporate from the resist layer during the drying process.